The AA8V 6AG7 Amplifier
by Greg Latta, AA8V

Interior Photos

Interior Photos of the Finished Amplifier:

Overall View: In this overall view of the amplifier wiring the front is at the left and the rear is at the right. At the left, mounted on the front of the amplifier, from bottom to top, are the key jack, on/off switch, band switch, and pilot light. Leads running from the bandswitch to the coil socket are kept as short as possible to minimize stray inductance. Wires to the plate tuning and load controls pass through the chassis via grommeted holes and are also kept as short as possible. The socket near the center of the chassis is for the 6AG7 amplifier tube. All connections are kept as short as possible. The green plate RF choke obscures the 47k screen dropping resistor and some of the other connections to the 6AG7 socket. Directly to the right of the RF choke are the 120k bleeder resistor and the blue power supply filter capacitors. The power supply filter choke, which looks like a small transformer, is slightly to the left at the bottom of the photo. At upper right is the socket for the 5Y3 rectifier tube, and directly below the 5Y3 socket is the power transformer, partly hidden by the blue filter capacitors. At the right, mounted on the back panel, from bottom to top, are the line cord, fuse, input connector and output connector. Coaxial cable is used to connect the input and output connectors to the appropriate parts of the circuit. The use of unshielded wire could lead to instability and unwanted parasitic oscillations. At the extreme upper right is the green output RF choke. Compare the wiring in this photo to the wiring in the prototype. (In the prototype, little attention was given to proper lead dress or soldering techniques.)	Click on the image for a larger view. Click here for a super detailed view.
Band switch and Coil Socket Wiring: In this close-up of the band switch and tank coil socket the signal runs to the band switch along the long curved black wire at the bottom of the photo. The right end of the wire is connected to a tie point with the 0.0047uf plate coupling capacitor. A wire from the tie point also runs through a grommeted hole in the chassis to the 100pf plate tuning capacitor. The band switch is a 3 pole/4 position rotary switch, but only one pole is used. Different pins on the coil socket connect to various taps on the tank coil. The band switch lessens the inductance of the tank coil as it is rotated clockwise by progressively shorting out larger portions of the coil. Black spaghetti is used on the wires to keep them from shorting against each other. A coaxial cable running to the output connector is attached to the top pin on the coil socket. A wire from this pin also runs through a grommeted hole in the chassis to the loading capacitor.	Click on the image for a larger view. Click here for a super detailed view.
6AG7 Socket and Filter Capacitor Wiring: The 6AG7 socket is near the middle of the chassis. In this close-up the front panel is to the left and the rear panel is to the right. Pin 1 on the tube socket is just to the left of the blue resistor. Pin numbers increase in the clockwise direction. At the top of the photo a coaxial cable brings the signal in from the input connector on the back panel. Coaxial cable should always be used for such connections, otherwise stray coupling and instability can occur. The green plate RF choke dominates the center of the photo. The blue 180 ohm cathode bias resistor (brown-gray-brown) can be seen near the bottom end of the RF choke. The large brown resistor to the right of the RF choke is the 120k ohm bleeder resistor (brown-red-yellow). The large blue capacitors are the 47uf power supply filter capacitors. The large yellow wire attached to the bottom filter capacitor runs to the center tap of the 5V filament winding on the power transformer. The large twisted green wires connected to pins 2 and 7 of the tube socket run to the 6.3V filament winding on the power transformer. The 47k screen dropping resistor is underneath the RF choke and is not visible in the photo. The brown 100k ohm grid resistor (brown-black-yellow) is on the left side of the tube socket. The ceramic disc capacitor at the bottom of the RF choke is the 0.0047uf plate coupling capacitor. It is connected to a tie point to its left where a curved black wire carries the output signal to the band switch. Another wire connected to the tie point runs through a grommeted hole in the chassis to the plate tuning capacitor. Note how all connections are kept as short as possible to minimize stray inductance and capacitance.	Click on the image for a larger view. Click here for a super detailed view.
5Y3 Socket and Input/Output Connector Wiring: The 5Y3 rectifier socket is mounted near the back of the chassis. In this photo the rear panel is on the right. The green RF choke is the output RF choke. This choke keeps the DC potential across the output loading capacitor and antenna zero at all times. The red wires connected to the 5Y3 socket are the high voltage leads from the power transformer, and the yellow leads are from the 5V filament winding. The rectified high voltage is taken from the center tap on the filament winding, which can't be seen in this photo. The SO-239 output connector is on the top at right, and the RCA input connector is at bottom right. Coaxial cables carry the signals to and from the back panel to the rest of the circuit. These connections must be shielded to avoid stray coupling and instability. Notice how the bottom cable shield is connected to the RCA input jack: The shield is trimmed back and a wire is wrapped around the shield and carefully soldered to it. The other end of the wire is then connected to the jack. This is the proper method for connecting a cable shield to a jack that is otherwise too small to accept it.	Click on the image for a larger view. Click here for a super detailed view.
Power Switch, Key Jack, and Power Supply Choke: The power switch and key jack are mounted on the front panel of the amplifier, which is to the left in the photo. The key jack is wired so that it keys the amplifier when when no plug is inserted in the jack. The key jack is bypassed with a 0.001uf capacitor to prevent any stray RF from entering or leaving through the jack. The power supply filter choke is at the bottom of the photo. This is a 65mA, 8H choke made by the SNC Manufacturing Co. The ceramic disc capacitor partly visible at the top of the photo is the 0.0047uf plate coupling capacitor.	Click on the image for a larger view. Click here for a super detailed view.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu